1. Field of the Invention
The invention relates to a mobile communication terminal such as a cellular phone, including therein a global positioning system module which is necessary to generate an accurate frequency or calibrate a frequency into an accurate frequency, and a method of calibrating a frequency in such a mobile communication terminal.
2. Description of the Related Art
Recently, in a field of mobile communication, both of a service provider and a service receiver wait for a service of providing data indicative of a position of a user. If a user could know not only where he/she is and/or where his/her friends are, but also geographic data such as shops or transportation around where a user is, data about entertainment such as which movie a user can see now and what time a movie starts, and/or news about traffics, richer communication society can be accomplished.
A principle of detecting a position of a user or others is in measurement of a time necessary for a radio signal to reach a terminal to thereby measure a distance between a first terminal which transmits a radio signal and a second terminal which receives the radio signal transmitted from the first terminal.
In order to measure a time necessary for a radio signal to reach a terminal, the terminal would be necessary to have a function of transmitting a signal having an accurate frequency or calibrating a frequency of a signal into an accurate frequency. This is because a distance is much influenced by the accuracy.
However, an oscillator which transmits a signal having a frequency with high accuracy is large in size, and further, a terminal including such an oscillator is high in fabrication cost.
Even if a mobile communication terminal had an oscillator which transmits a signal having a frequency with high accuracy, a frequency of a signal transmitted from the oscillator would be fluctuated due to variance in an ambient temperature and environmental conditions.
Hence, it would be more practical for a mobile communication terminal to calibrate a frequency than to have a function of transmitting a signal having an accurate frequency. Even so, it is necessary for a mobile communication terminal to be able to transmit a signal having a frequency with high accuracy, even if frequency is calibrated.
A mobile communication terminal generally has its inherent function of calibrating a frequency of a signal transmitted therefrom, regardless of whether it includes a GPS module. However, an accuracy of a calibrated frequency generally cannot satisfy an accuracy required in GPS.
Hereinbelow are explained an auto-frequency calibration (AFC) and frequency calibration control (FCC) both of which a mobile communication terminal generally has. Auto-frequency calibration (AFC) means a function of automatically calibrating a frequency, and frequency calibration control (FCC) means a function of calibrating a frequency, inherent to a GPS module.
More specifically, auto-frequency calibration is defined as a function of, when a base station including an oscillator transmitting a signal having an accurate frequency makes connection with a terminal, calibrating an operational frequency of the terminal, based on a signal transmitted from the oscillator.
Auto-frequency calibration (AFC) is described in Japanese Patent Application Publications Nos. 8-56153 and 11-355102, for instance.
FIG. 1 is a block diagram of an auto-frequency calibration unit 100 having a function of auto-frequency calibration.
The auto-frequency calibration unit 100 is comprised of a down converter (D/C) 102 which receives a radio signal transmitted from a base station, through an antenna (not illustrated), converts a frequency of the received signal, and outputs an intermediate frequency (IF) signal, an auto-frequency calibration (AFC) circuit 103 which counts the number of the intermediate frequency signals transmitted from the down converter 102, and outputs a voltage in accordance with the counts, a digital-analog (D/A) converter circuit 104 which converts a voltage transmitted from the auto-frequency calibration circuit 103, into analog data, and an oscillator 105 (TCXO1) which generates an operation frequency of a terminal.
FIG. 2 is a flow chart of auto-frequency calibration carried out by the auto-frequency calibration unit 100 illustrated in FIG. 1.
After a terminal has established connection with a base station in step S501, auto-frequency calibration starts in step S502.
The auto-frequency calibration circuit 103 counts the intermediate frequency (IF) signals for a predetermined period of time, in step S503. Hereinbelow, a count of the intermediate frequency signals is called a real count.
The predetermined period of time during which the IF signals are counted is determined based on a frequency of a signal transmitted from the oscillator 105. Hence, if a signal transmitted from the oscillator 105 has an inaccurate frequency, the predetermined period of time is also inaccurate. Hereinbelow, a count of the IF signals counted by the auto-frequency calibration circuit 103 when a signal transmitted from the oscillator 105 has an accurate frequency is called an ideal count.
Then, the auto-frequency calibration circuit 103 calculated a difference between the real count and the ideal count, in step S504.
Then, the auto-frequency calibration circuit 103 judges whether the thus calculated difference between the real count and the ideal count is within a predetermined allowable range, in step S505.
If the difference is out of the predetermined allowable range (NO in step S505), the auto-frequency calibration circuit 103 carries out calibration. Specifically, the auto-frequency calibration circuit 103 calculates a control voltage to be input into the oscillator 105, based on the difference, in step S506.
The thus calculated voltage is input as a digital signal into the digital-analog converter circuit 104. The digital-analog converter circuit 104 converts the voltage into an analog signal, and then, outputs the analog signal to the oscillator 105 as a control voltage in accordance with which the oscillator 105 is controlled.
Then, the oscillator 105 outputs a signal having an accurate frequency, and the down converter 102 receives the signal from the oscillator 105. As a result, the down converter 102 can transmits an intermediate frequency signal having a more accurate frequency.
The steps S503, S504 and S505 are repeatedly carried out until a difference between the real count and the ideal count becomes within the predetermined allowable range. If the difference becomes with the predetermined allowable range (YES in step S505), the auto-frequency calibration circuit 103 finishes the calibration, and starts next calibration.
A condition in which a real count of the IF signal is within a predetermined allowable range, and hence, the auto-frequency calibration circuit 103 does not carry out calibration is called a lock-up condition.
A lock-up condition means that a frequency of a signal transmitted from the oscillator 105 is calibrated.
The auto-frequency calibration circuit 103 continues counting the IF signals even in a lock-up condition. If the difference is out of the predetermined allowable range (NO in step S505), the auto-frequency calibration circuit 103 restarts the calibration mentioned above.
The above-mentioned predetermined allowable range with which a difference between the real count and the ideal count is compared corresponds to an accuracy of a frequency in a lock-up condition of auto-frequency calibration.
Hereinbelow is explained frequency calibration control (FCC).
Frequency calibration control (FCC) means a function of calibrating a frequency, inherent to a GPS module, more specifically, a function of outputting data in accordance with which an operational frequency of a GPS module is calibrated, based on an input signal having an accurate frequency. Frequency calibration control (FCC) is described in Japanese Patent Application Publication No. 2000-506348 based on PCT/US97/03512 (WO97/33382) or Japanese Patent Application Publication No. 11-513787 based on PCT/US96/16161 (WO97/14049), for instance.
FIG. 3 is a block diagram of a frequency calibration control unit 200 having a function of frequency calibration control (FCC).
The frequency calibration control unit 200 is comprised of an oscillator (TCXO2) 201 which generates and transmits a frequency signal having a frequency equal to an operational frequency of the frequency calibration control unit 200, a frequency calibration control (FCC) counter 202 which receives a reference clock signal transmitted from an external circuit (not illustrated) and the frequency signal transmitted from the oscillator 201, and counts the number of the reference clock signals, a frequency calibration control operation unit 203 which receives a count transmitted from the frequency calibration control (FCC) counter 202 and the frequency signal transmitted from the oscillator 201, and a GPS signal processor 204 which receives a signal transmitted from the frequency calibration control (FCC) operation unit 203 and the frequency signal transmitted from the oscillator 201.
Since a later mentioned GPS module operates in accordance with the frequency signal transmitted from the oscillator 201, parts constituting the frequency calibration control unit 200 operate in accordance with a frequency indicated in the frequency signal transmitted from the oscillator 201.
The reference clock signal to be input into the FCC counter 202 has an accurate frequency based on which calibration is carried out.
FIG. 4 is a flow chart showing an operation of the frequency calibration control unit 200.
The frequency calibration control starts, in step S601. The FCC counter 202 counts the number of the reference clock signals for a predetermined period of time, in step S602.
The predetermined period of time during which the reference clock signals are counted is determined based on a frequency of a signal transmitted from the oscillator 201. Hence, if a signal transmitted from the oscillator 201 has an inaccurate frequency, the predetermined period of time is also inaccurate.
The FCC operation unit 203 calculates a difference between a real count and an ideal count of the FCC counter 203, in step S603, and outputs the difference to the GPS signal processor 204, in step S604. The difference is called “calibration data” hereinbelow.
Thus, the frequency calibration control ends, in step S605.
FIG. 5 is a block diagram of a mobile communication terminal 300 including the auto-frequency calibration unit 100 illustrated in FIG. 1 and the frequency calibration control unit 200 illustrated in FIG. 3.
As illustrated in FIG. 5, a communication antenna 106 is connected to the down converter 102 of the AFC unit 100, and a GPS antenna 205 is connected to the GPS signal processor 204 of the FCC unit 200 as a GPS module. The AFC unit 100 and the FCC unit 200 are electrically connected to each other such that a frequency signal transmitted from the oscillator (TCXO1) 105 is input into the FCC counter 202. In other words, the FCC counter 202 receives the frequency signal from the oscillator 105 as the reference clock signal.
If the AFC unit 100 is in a lock-up condition and the oscillator 105 transmits a signal having a frequency accurate to some degree, the FCC operation unit 203 and the FCC counter 202 are driven to thereby transmit the calibration data to the GPS signal processor 204.
An effective accuracy with which calibration is carried out in accordance with the frequency calibration control (FCC) is almost equal to or smaller than an effective accuracy with which calibration is carried out in accordance with the auto-frequency calibration (AFC).